Ecological and individualistic fallacies in health disparities research

Ecological fallacy, defined by Robinson in 1950 as incorrect inferences about individuals based on characteristics and associations observed among groups (1), is a well-recognized concept in epidemiology and statistics.Research has shown when aggregate values for variables of interest (eg, median area-level income) were used as proxies for individual-level variables (eg, household income), covariates estimated from regression models may be biased (2,3) and the sign of regression coefficients could change (4).Policy makers and health-care providers who rely on these estimations could inadvertently draw the wrong conclusions or target the wrong group for interventions.Health disparities research, including cancer disparities research, using observational data from registries, medical records, or administrative claims often lacks information on individual-level socioeconomic status (SES) variables, such as income, educational attainment, and employment status.Many studies use aggregate statistics at selected geographic units (eg, county, zip code, or census tract) as a substitute for individuallevel SES.This approach, known as the census-based approach (5-7), is common practice in disparities research, where these variables are treated as "proxies" for individual SES and interpreted as if SES had been measured among individuals.Although some studies acknowledge ecological fallacy as a limitation, the use of the census-based approach is widely accepted by researchers as well as peer reviewers and is frequently viewed as an inevitable compromising analytical strategy driven by the lack of individual-level data on SES variables.Davis et al. ( 8) made an important contribution to the literature of cancer disparities research by identifying appropriate data for neighborhood and individual income to showcase the issue of ecological fallacy when linking area-level factors to individual outcomes.The authors documented poor agreement between neighborhood and individual income measures, especially in rural communities (8).In addition, associations between neighborhood income and survival among patients with colorectal cancer were much smaller than associations of individual income and survival (8), suggesting that misclassification bias from using neighborhood income as a proxy for individual income may contribute to an underestimation of the income effect.Findings from this study serve as a cautionary tale for